There have been many attempts in the prior art to increase the utilization of the heat retained in a source of steam, in order to produce power. Two-phase geothermal steam has been shown to be a convenient and readily available source of power producing steam in many areas of the world.
In one method, water and steam are separated at a wellhead of geothermal fluid, and the two fluids are utilized in separate power plants. However, the thermodynamic efficiency of a power plant operating on geothermal water may be too low to warrant the capital cost of the equipment.
U.S. Pat. No. 5,088,567 discloses a method for utilizing separated geothermal water and geothermal steam in a single power plant. The geothermal water preheats the working fluid before the latter is introduced to a vaporizer, from the condenser cooled temperature to the temperature just below that of the vaporizer. The geothermal steam heats the working fluid within the vaporizer at conditions of constant temperature and pressure. The vaporized working fluid is expand in a heat engine and the heat-depleted working fluid is condensed to produce condensate which is returned to the vaporizer.
U.S. Pat. No. 5,660,042 discloses a similar method for using two-phase liquid in a single Rankine cycle power plant, and vaporized working fluid is applied in parallel to a pair of turbine, one of which may be a steam turbine.
U.S. Pat. No. 5,664,419 discloses the use of a vaporizer, preheater, and recuperator. The vaporizer produces vaporized organic fluid to be expanded in the tubing and cooled geothermal steam. The preheater transfers sensible heat to the organic fluid from separated geothermal brine and from steam condensate from the vaporizer. The recuperator, which receives organic vapor exhausted from the turbine, permits additional heat to be used by the organic working fluid by heating condensed organic liquid pumped to the vaporizer through the recuperator and preheater.
The use of a recuperator also allows heat to be more efficiently transferred from the geothermal steam to the organic working fluid. The evident heat transfer from the geothermal steam to the organic working fluid is reflected by the similarity of the heat transfer rate of the working fluid with respect to that of geothermal steam. As shown in FIG. 1, which is a temperature T/heat Q diagram of both the working fluid and the geothermal steam, the heat transfer rate of the organic working fluid and of the geothermal steam is substantially similar. Curve 5 represents the heat transfer rate of the geothermal fluid as it enters the vaporizer and exits the preheater at point A, while curve 6 represents the heat transfer rate of the organic working fluid. The inclined portion of curve 6 from the condenser temperature and rising to point E, which is the boiling temperature of the working fluid, represents the sensible temperature rise of the working fluid as it flows through the preheater and vaporizer. Q2 represents the amount of heat input to the working fluid. The break point, or the discontinuity, of working fluid curve 6 is shown to be vertically below that of geothermal fluid curve 5, and therefore heat is efficiently transferred to the working fluid. As the gap between corresponding points of curves 5 and 6 increases, more heat is dissipated and less heat is transferred to the working fluid from the geothermal fluid. For purposes of comparison, curve 1 represents the heat transfer rate of working fluid of a power plant provided without a recuperator as it rises from the condenser temperature to point D flowing a heat input of Q1. The use of the recuperator therefore increases the heat input by an amount of Q2-Q1.
At times, the liquid content of the geothermal fluid is not significantly high, and geothermal-based power plants are forced to use a portion of the high-temperature and high-pressure geothermal steam to preheat the organic working fluid, resulting in ineffective heat utilization.
There is therefore a need to provide a geothermal-based power plant system for producing power with a relatively efficient rate of heat transfer from geothermal fluid having a relatively low liquid content to organic working fluid.
It is an object of the present invention to provide a geothermal-based power plait system for producing power with a relatively efficient rate of heat transfer from geothermal fluid having a relatively low liquid content to organic working fluid.
It is an additional object of the present invention to provide a method for achieving a similar heat transfer rate of the working fluid as that of geothermal fluid when the power plant system utilised geothermal fluid has a relatively low liquid content.
Other objects and advantages of the invention will become apparent as the description proceeds.